Comparison of Equilibrium Climate Sensitivity Estimates From Slab Ocean, 150‐Year, and Longer Simulations.

We compare equilibrium climate sensitivity (ECS) estimates from pairs of long (≥800‐year) control and abruptly quadrupled CO2 simulations with shorter (150‐ and 300‐year) coupled atmosphere‐ocean simulations and slab ocean models (SOMs). Consistent with previous work, ECS estimates from shorter coupled simulations based on annual averages for years 1–150 underestimate those from SOM (−8% ± 13%) and long (−14% ± 8%) simulations. Analysis of only years 21–150 improved agreement with SOM (−2% ± 14%) and long (−8% ± 10%) estimates. Use of pentadal averages for years 51–150 results in improved agreement with long simulations (−4% ± 11%). While ECS estimates from current generation U.S. models based on SOM and coupled annual averages of years 1–150 range from 2.6°C to 5.3°C, estimates based longer simulations of the same models range from 3.2°C to 7.0°C. Such variations between methods argues for caution in comparison and interpretation of ECS estimates across models. Plain Language Summary: Precise definition and estimation of equilibrium climate sensitivity (ECS) continues to challenge model intercomparison. While annual analyses of years 1–150 of coupled atmosphere‐ocean models agree with slab ocean model simulations, they underestimate coupled ECS estimates from multicentennial to millennial scale simulations. However, long‐term ECS estimates can be largely recovered through a combination of (1) ignoring the first 50 years of abrupt 4 times preindustrial CO2 simulation dominated by early timescales of ocean response and (2) using pentadal (5‐year) averages instead of annual ones for years 51–150. This variation between methods argues for reconsideration of ECS estimation and application acknowledging that slab ocean estimates systematically ignore potential sources of enhanced sensitivity and simulations longer than 150 years are necessary for precise estimation of the long‐term trend. Key Points: Equilibrium climate sensitivity (ECS) estimates for a single coupled model can vary by more than 1°C (20%) depending on analysis methodECS estimates from ≥300‐year coupled simulations from current U.S. models range from 3.1°C to 7.0°C, another method giving 2.7°C to 5.3°CAnalysis of years 21–150 agrees with slab ocean ECS, but pentadal analysis of years 51–150 reduces bias against long, coupled simulations [ABSTRACT FROM AUTHOR]